organic nanodots for multiphotonics: synthesis and...

Organic nanodots for multiphotonics: synthesis and photophysical

studiesw

Olivier Mongin,a Anna Pla-Quintana,b Francesca Terenziani,ac Delphine Drouin,a

Celine Le Droumaguet,aAnne-Marie Caminade,

bJean-Pierre Majoral*

band

Mireille Blanchard-Desce*a

Received (in Montpellier, France) 15th February 2007, Accepted 31st May 2007

First published as an Advance Article on the web 8th June 2007

DOI: 10.1039/b702452p

Organic nanodots based on the gathering of an exponentially increasing number of two-photon

fluorophores on a dendritic platform of controlled size and symmetry represent a promising non-

toxic alternative to quantum dots for (bio)imaging purposes. This modular route offers a number

of advantages in terms of versatility but also raise a number of questions to be addressed. In

particular, possible interactions between fluorophores, due to confinement effects, have to be

taken into account. With this aim in mind we have investigated and compared the photophysical

and two-photon absorption (TPA) properties of two series of organic nanodots of different

geometries: spherical-like organic nanodots derived from a dendritic scaffold built from a

cyclotriphosphazene core and dumbbell-like organic nanodots derived from a dendritic scaffold

built from an elongated rod-like chromophore. The study provides evidence that the different

topology and nature of the dendritic architecture lead to significant changes in photoluminescence

characteristics as well as to subtle variations of the TPA efficiency. As a result, the dumbbell-like

nanodots although less promising in terms of two-photon induced fluorescence (due to partial

quenching of fluorescence efficiency) also demonstrate that improvement of the TPA efficiency can

be achieved by playing on the nature and topology of the dendritic scaffold of the nanodots.

Introduction

Molecular two-photon absorption (TPA) has attracted a lot of

interest over the last decade owing to its applications in

various fields including spectroscopy,1,2 three-dimensional

optical data storage,3–5 microfabrication,6–8 laser up-conver-

sion,9,10 high-resolution three-dimensional imaging of biolo-

gical systems,11–13 and photodynamic therapy.14 Among these,

two-photon-excited fluorescence (TPEF) has gained wide-

spread popularity in the biology community and given rise

to the technique of two-photon laser scanning fluorescence

microscopy.11–13 Using a two-photon excitation process (i.e. a

nonlinear process involving the simultaneous absorption of

two photons) instead of a conventional one-photon excitation

offers a number of advantages. These include the ability for a

highly confined excitation (and intrinsic three-dimensional

resolution) and increased penetration depth by replacing

typical one-photon excitation in the UV–visible blue region

by two-photon excitation in the visible red–NIR region, owing

in particular to the reduction of scattering losses.

Most of these applications strongly benefit from the use of

chomophores displaying very high TPA cross-sections in

order to significantly decrease the excitation intensity or

provide improved excitation efficiency and selectivity. In-

deed, in recent years a considerable effort has been devoted

to the design and investigation of chromophores with large

TPA cross-sections, exploring in particular dipolar10,15–20

and quadrupolar,17,21–38 structures. Lately, attention has

turned towards multipolar39–43 and conjugated branched

structures44–51 including dendrimers.52–55

Depending on the targeted applications, two-photon chro-

mophores also have to satisfy additional requirements. For

instance, the use of such chromophores for biological multi-

photonic imaging calls for the design of fluorophores combin-

ing a high fluorescence quantum yield (F) and TPA cross-

section (s2) in the spectral range of interest (i.e. 700–1200

nm)56 several orders of magnitude larger than endogenous

chromophores (whose TPA cross-sections range between 10�5

to about 1 GM).13 This would open the route for selective

excitation of two-photon chromophores leading to signifi-

cantly improved signal to noise ratio (reduction of back-

ground fluorescence) and reduction of photodamage.57 Low

(photo)toxicity and high photostability are important addi-

tional criteria.

Recently semiconductor nanocrystals (quantum dots:

QDs) have been shown to provide a particularly effective

approach to fluorescent nano-objects with extremely large

a Synthese et Electrosynthese Organiques (CNRS, UMR 6510),Institut de Chimie, Universite de Rennes 1, Campus Scientifique deBeaulieu, Bat. 10A, F-35042 Rennes Cedex, France. E-mail:[email protected]; Fax: + 33 2 23 23 62 77

b Laboratoire de Chimie de Coordination, CNRS, 205 route deNarbonne, F-31077 Toulouse Cedex 4, France. E-mail: [email protected]; Fax: +33 5 61 55 30 03

cDipartimento di Chimica GIAF, Universita di Parma, Parco Areadelle Scienze 17/A, 43100 Parma, Italyw This paper was published as part of the special issue on Dendrimersand Dendritic Polymers: Design, Properties and Applications.

1354 | New J. Chem., 2007, 31, 1354–1367 This journal is �c the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007

PAPER www.rsc.org/njc | New Journal of Chemistry

TPA cross-sections (s2F up to 47 000 GM)58 overcoming the

TPA responses of molecular two-photon fluorophores. QDs

have gained tremendous popularity for imaging applications

due to their intense, tunable (e.g. depending on the size and

composition) and robust photoluminescence properties.59

However, these inorganic systems suffer from several draw-

backs such as biological toxicity60 (due in particular to the

presence of heavy metals such as cadmium) and blinking. In

addition their surface functionalization for different pur-

poses such as targeting, recognition, conjugation to biomo-

lecules or specific labeling is possible but not undemanding.

These inorganic nano-objects also raise a number of ques-

tions with respect to environmental issues. The implementa-

tion of alternative strategies towards ‘‘soft’’ and

biocompatible substitutes is thus of high interest. Recently,

we have developed a promising route towards all-organic

nanodots showing competitive two-photon brightness (com-

parable to the highest reported for QDs).61 This route is

based on the confinement of a large number of two-photon

fluorophores showing significant TPA cross-sections within

single spherical nano-objects. Such nanodots can be ob-

tained by grafting two-photon (TP) fluorophores on the

surface of a dendrimeric platform (Fig. 1). This modular

strategy offers several potential advantages: photolumines-

cence (PL) characteristics can, in principle, be tuned by

playing on the nature of the fluorophore, while the dendri-

mer scaffold can be chosen so as to minimize toxicity effects

and control clearance ability.62 However, implementing such

modular approach also requires taking into account inter-

actions between the various molecular building blocks

including possible interactions between decorating fluoro-

phores. The confinement of fluorophores within the nanodot

imposes close proximity between chromophores which fa-

vors interchromophoric interactions: (i) in the ground state

with potential effects on the absorption characteristics and

(ii) in the excited state with impending marked effects on PL.

Such interactions can significantly affect the optical re-

sponses and photophysical properties of the nanodots which

will then differ from the mere addition of the contributions

of isolated single fluorophores. For instance formation of

dimers or aggregates between adjacent fluorophores could

lead to fluorescence quenching.63 Also interactions in the

excited state can lead to changes of PL characteristics due to

various phenomena (energy transfer, excimer formation or

exciton annihilation which is an important decay process in

chromophore decorated dendrimers at high excitation in-

tensity64).

Also it has been shown that interactions between chromo-

phores within dimers65 or aggregates36,66 can lead to signifi-

cant change in TPA responses. Such effects are expected to be

strongly dependent on both the nature of the chromophores

and their proximity and relative orientations. In this respect

the geometry and symmetry of the dendritic scaffold are

expected to play an important role.

In this prospect, we have investigated and compared the

photophysical and TPA properties of two series of organic

nanodots of different geometries: spherical-like organic nano-

dots (SOND, Fig. 1) and dumbbell-like organic nanodots

(DOND, Fig. 2) built from the gathering of an exponentially

increasing number of two-photon active fluorophores F on the

periphery of dendrimeric platforms of different symmetry. The

SOND series is the prototypical organic dots series that was

recently prepared as a proof of concept test series.61 In this

work, we examine how the two-photon absorption and PL

properties are affected by the nature and symmetry of the

dendrimeric scaffold. The SOND series G1–G4 is derived from

a dendritic scaffold built from a C3 symmetry cyclotriphos-

phazene core whereas the DOND series G10–G02 is derived

from a dendritic scaffold built from an elongated rod-like

fluorophore. The different symmetry of the cores induces a

difference in both the number and the symmetry of the

arrangement of the two-photon fluorophores on the surface

of the organic nanodot. In the case of SOND series, the two-

photon fluorophores on the surface experience similar envir-

onment in relation with the symmetry of the core of the

dendrimeric scaffold which controls the overall geometry

and topology (Fig. 1). In contrast, the decorating fluorophores

of the DOND series clearly experience inhomogeneous envir-

onments due to different nature and topology of the core (Fig.

2). This can play a significant role in terms of interchromo-

phoric interactions. If this is the case, the PL characteristics of

compounds of the SOND and DOND series can significantly

differ for a similar number of decorating fluorophores but a

different topology. This is expected to affect the TPEF effi-

ciency of the nanodot. Another important issue with respect to

the TPEF efficiency is the variation of the overall TPA cross-

Fig. 1 Schematic representation of homochromophoric spherical-type organic nanodots (SOND).

This journal is �c the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007 New J. Chem., 2007, 31, 1354–1367 | 1355

section as a function of the topology. Additive contributions of

the two-photon fluorophores to the overall TPA efficiency of

the nanodot was observed in the case of the SOND series.61

Since the environment and the packing of the decorating

fluorophores may differ in the two series, modification of their

TPA efficiency cannot be excluded. Finally whereas in the case

of the SOND series, energy transfer can occur among similar

fluorophores on the periphery (excitation energy migration),

energy transfer can in principle (depending on the nature and

spectroscopic characteristics of the core chromophore) occur

both along the surface (homotransfer) and between the core and

the surface of the nanodot (heterotransfer) in the DOND case.

Results and discussion

Synthesis

The synthesis of the spherical-like first generation dendrimer

G1 was carried out by grafting 12 equivalents of the TPA

chromophore F to the P(S)Cl2 end groups of the first genera-

tion phosphorus dendrimer67–69 built from a cyclotriphospha-

zene core70 (Scheme 1). The same reaction carried out with the

second, third and fourth generations of the same series of

dendrimers afforded the spherical dendrimers G2, G3 and G4,

respectively (Fig. 3), as previously reported.61

Two series of dumbbell-like dendrimeric compounds were

synthesized, both issued from the same biphenolic chromo-

phore 271 (Scheme 2) as the starting compound. The first series

(C0, C1) was synthesized to serve as model for the photo-

physical properties of the core fluorophores of the other series

(i.e. DOND series). The synthesis of the generation 0 of the

monochromophoric series C0 necessitates first the isolation of

the pentasubstituted cyclotriphosphazene 1, which is then

reacted with chromophore 2. The substitution of the sixth Cl

of the cyclotriphosphazene with such bulky substituent is slow

and needs 7 days to go to completion (Scheme 2). 31P NMR is

particularly suitable to monitor this reaction; indeed the

Fig. 2 Schematic representation of heterochromophoric dumbbell-like organic nanodots (DOND).

Scheme 1 Synthesis of spherical dendrimer G1.

Fig. 3 Structures of spherical-type dendrimers G2–G4.


spectrum of compound 1 displays an ABB0 system, whereas

the sixth substitution induces an AA0A00 system for C0,

affording a narrow multiplet.

The generation 0 of the multichromophoric series of dumb-

bell-like dendrimers (i.e. DOND series) is synthesized using a

different route. Indeed, in this case N3P3Cl6 (large excess) is

reacted with the biphenol 2 to afford compound 3.71 In the

second step, 10 equivalents of the two-photon fluorophore F61

are grafted to afford after a prolonged heating (37 days) the

generation 0 of the dumbbell-like multichromophoric dendri-

mer G00 (Scheme 3). In this case also, 31P NMR is an

invaluable tool, since the completion of the reactions is shown

by the appearance of a narrow multiplet, observed after a

series of intermediate compounds displaying ABB0 type

spectra.

First generation compounds of both monochromophoric

and multichromophoric dumbbell-like dendrimers are synthe-

sized by grafting phenols to the P(S)Cl2 end groups of

dendrimer 4.71 The monochromophoric model compound

C1 is obtained by reaction with HO–C6H4–OMe, whereas

the multichromophoric compound G01 is obtained by reaction

with fluorophore F (Scheme 4, Fig. 2). In both cases, the

narrow multiplet (due to the unsymmetrical substitution on

the N3P3 groups) observed at 62.4 ppm for the P(S)Cl2 end

groups by 31P NMR is replaced by another narrow multiplet

at 64.5 ppm. An intermediate signal observed at 69.4 ppm and

corresponding to the monosubstitution (P(S)ClOAr) totally

disappears when the reaction is over. The same method is

applied for the synthesis of the second generation of the

dumbbell-like multichromophoric dendrimer G02. This com-

pound possesses two types of chromophoric units: one at the

core and 40 as end groups (Scheme 5, Fig. 2).

Photophysical properties

One-photon absorption. All dendrimers show a strong ab-

sorption band in the near UV–visible-blue region (Table 1).

The normalized absorption spectra of the multichromophoric

dendrimers of the SOND series overlap almost perfectly with

the absorption spectrum of the isolated decorating fluoro-

phore F with only a slight broadening at the red tail (Fig. 4).

The molar extinction coefficients increase almost linearly with

the number of decorating fluorophores leading to giant ab-

sorption coefficients for the G4 SOND dendrimer (Table 1).72

These trends suggest that no noticeable interactions occur in

the ground state.

The absorption spectra of dendrimers of the dumbbell-like

series (DOND dendrimers) are broader and slightly blue-

shifted with respect to that of the SOND dendrimers (Table

1, Fig. 4). The absorption spectra of the core chromophores

(C0 for G00 and C1 for G01 and G02) mostly overlap, have

comparable extinction coefficient and are only slightly blue-

shifted with respect to that of peripheral fluorophores F (Table

1, Fig. 5). However, comparison of the absorption spectra of

dendrimers of the DOND series with that corresponding to

that expected for an additive contributions of core and per-

ipheral chromophores F (Fig. 6) shows that the assembly of

two-photon fluorophores F in the DOND series leads to

broader spectra and lower effective absorption coefficient

per fluorophore. This indicates a larger inhomogeneous

Scheme 2 Synthesis of monochromophoric model C0.

Scheme 3 Synthesis of dumbbell-like multichromophoric compound G00 (only half of the molecule is represented for G00).


broadening suggesting that the decorating fluorophores of the

DOND series experience a larger range of environments (and

disorder) in relation with the different geometry and lower

symmetry of their arrangement within the nanodots.

Fluorescence. The emission spectra of dendrimers of the

SOND series clearly show a change of the emission band shape

and position with increasing generation number (Fig. 4). A

change of the vibronic relative intensities is observed along

with a red-shift of the emission maxima (Table 1). Whereas the

0–0 transition is clearly the more intense vibronic band in the

emission spectrum of the parent isolated fluorophore F, the

0–1 transition becomes more intense in the dendrimers and

this effect becomes stronger on going from G1 to G2 com-

pounds (Fig. 4). The change in vibronic structure reveals an

increase in reorganization energy in the dendrimers as com-

pared to the isolated fluorophore, indicating that the emitting

fluorophore has stronger interactions with its environment

when gathered on the surface of the dendrimer. Indeed the

proximity of adjacent chromophores modifies the local envir-

onment of the emitting fluorophore as compared to an isolated

fluorophore surrounded only by solvent molecules. The loss of

fine vibronic structure, red-shift and spectral broadening of the

emission band with increasing generation number suggests

that the polarity of the environment of the emitting fluoro-

phores is increasing. It should be stressed that the emission of

quadrupolar chromophore F is very sensitive to polarity73

allowing its use as a probe of its local environment. Fig. 7

shows emission spectra calculated by the aid of the model

reported in ref. 73, for different environmental reorganization

energies (eor). Parameters used are the same as reported in ref.

73, since the chromophore is the same. In the adopted model,

the evolution of position and shape (together with inhomoge-

neous broadening effects) of the emission band of some

quadrupolar chromophores (belonging to class I as defined

in ref. 73), is ascribed to symmetry breaking in the relaxed

excited state. Polar environments stabilize broken-symmetry

states, corresponding to dipolar states. The dipolar nature

of the broken-symmetry relaxed excited state is thus respon-

sible for the solvatochromic behavior of fluorescence bands. In

Scheme 4 Synthesis of monochromophoric dendrimer C1 and dumbbell-like dendrimer G01 (only half of the molecule is represented for G01).

Scheme 5 Synthesis of dumbbell-like dendrimer G02 (only half of the molecule is represented for G02).


Fig. 7, the variation of eor nicely describes the evolution of the

emission spectrum in different dendrimer generations and

symmetries (this correspondence is summarized in the legend).

Increasing dendrimer generation corresponds to increased eorvalues: chromophores act as dielectrics, more polar than the

dissolving solvent (toluene). The effect of solvent polarity and

that of dendrimer generation/shape are very similar, on the

band position and shape. For example, we can estimate that

the polarity of the nanodots varies from lower than that of

CHCl3 for low-generation nanodots, up to almost that of

acetone for high-generation nanodots.

A distinct feature of the emission characteristics of dendri-

mers of the SOND series is their low anisotropy values. Parent

fluorophore F exhibits anisotropy of 0.18 in toluene, whereas

Table 1 Photophysical data for fluorophore F and dendrimers G1–G4 (SOND), C0–C1 (model dumbbell-like chromophores) and G00–G02(DOND) in toluene

No. of fluorophores MW/g mol�1 labs/nm e/M�1 cm�1 lem/nm Fa t/nsb rc s2 (max.)d/GM

F 1F 841 386 84 900 420 0.83 0.67 0.18 765G1 12F 11 485 385 1 004 000 423 0.75 0.71 0.03 8880G2 24F 24 102 386 2 035 000 426 0.71 0.69 0.02 17 700G3 48F 49 335 386 3 785 000 441 0.62 0.71 0.01 29 800G4 96F 99 804 386 7 101 000 445 0.48 0.66 0.01 55 900C0 1C0 2014 377 73 100 410 0.81 0.82 0.22 124C1 1C1 5377 378 71 500 412 0.68 0.85 0.28 146G00 1C0 + 10F 9184 382 805 000 443 0.44e 0.88f —g 7100G01 1C1 + 20F 19 698 382 1 537 000 444 0.11e 0.71f —g 14 300G02 1C1 + 40F 40 726 384 3 000 000 445 0.26e 0.65f —g 32 800

a Fluorescence quantum yield determined relative to fluorescein in 0.1 MNaOH. b Experimental fluorescence lifetime measured by time-correlated

single photon counting. c Steady-state fluorescence anisotropy (lexc = 382 nm). d 1 GM = 10�50 cm4 s photon�1. e Determined for lexc =

labs(max.). f Determined for lem = lem(max.). g Anisotropy is wavelength-dependent (see Fig. 9).

Fig. 4 Normalized absorption and emission spectra (lexc = labs(max.)) in toluene of SOND G1–G4, and DOND G00–G02. The absorption and

emission spectra of fluorophore F in toluene are also included for evaluation of the effect of the gathering of fluorophores F on the surface of

dendritic nanodots.


G1–G4 display much lower anisotropies, with values ranging

between 0.03 and 0.01 (Table 1). Since rotational diffusion of

the SOND dendrimers is expected to be much slower than that

of F due to their size (the radius of dendrimer G4 is estimated

to about 4 nm), the depolarization can be ascribed to fast

energy transfer between adjacent decorating fluorophores74–79

(excitation energy migration or homotransfer).80 We observe

that the fluorescence lifetimes remain roughly unchanged

(Table 1) which is consistent with the hypothesis of homo-

chromophoric energy transfer. Indeed homotransfer is possi-

ble via a Forster mechanism81 given the non-negligible overlap

between absorption and emission spectra of decorating fluoro-

phores. We also observe a continuous decrease of the fluores-

cence quantum yield with increasing generation number (Ta-

ble 1). This could be due to the occurrence of traps leading to

fluorescence quenching or formation of excimers (consistent

with an increasing red-tail) with lower fluorescence quantum

yields.82 The number of traps is expected to statistically

increase with increased generation number which could ex-

plain the variation of fluorescence quantum yield.

The analysis of the variation of the emission of dendrimers

of the DOND series is more intricate because their emission

stems from both core and peripheral fluorophores (Fig. 4). The

low-energy absorption bands of model core (C0, C1) and

peripheral (F) compounds show a major overlap (Fig. 5) and

similar intensities (Table 1) making selective one-photon ex-

citation of only one type of chromophore unattainable. In

addition, we observe that the emission spectra of model

compounds in toluene show a significant overlap, the emission

of fluorophore F being slightly red-shifted (Fig. 5). Emission

spectra of DOND dendrimers G00–G02 show significant differ-

ences as compared to their SOND analogues bearing similar

number of decorating fluorophores (i.e. G1–G3). The emission

band of DOND dendrimers G00–G02 are clearly red-shifted

and broadened (in particular in the red side) with respect to

their SOND analogues G1–G3 (Fig. 8). A possible explanation

is that the emitting fluorophores in the DOND dendrimers

experience a different environment than in the corresponding

SOND dendrimers. This could be attributed to the proximity

not only of adjacent identical decorating fluorophores but also

of the core chromophore. The decorating fluorophores located

close to the core (shown in hashed in Fig. 2) clearly experience

different environment. Simulations based on the model re-

ported in ref. 73 also indicate that DONDs are more polar

objects than corresponding SONDs (see Fig. 7), suggesting a

closer packing of chromophores in DONDs. This finds a

possible confirmation in the lower fluorescence quantum yield

of DONDs with respect to corresponding (i.e. containing

approximately the same number of chromophores) SONDs.

In addition, interchromophoric interactions in the excited

state (leading to formation of excimer, or hetero-excimer if

the core chromophore is involved) can be facilitated in the

case of the DOND series due to the larger inhomogeneous

broadening and difference (disorder) in packing of the

decorating fluorophores on the surface of the DOND den-

drimers. Such phenomenon could explain the even more

pronounced red tail in the emission band of dendrimers of

the DOND series (Fig. 8). Interestingly, the more pro-

nounced red tail is observed for G 01 which also gives rise

to the largest inhomogeneous broadening of the absorption

spectra (Fig. 4).

As opposed to the case of homochromophoric dendrimers

of the SOND series whose anisotropy values remain constant

in the whole emission range (400–600 nm), a continuous

Fig. 5 Normalized absorption and emission spectra (lexc = labs(max.)) in toluene of model chromophore F and model dendrimers

C0–C1.

Fig. 6 Comparison of experimental and theoretical (evaluated from

additive contributions of isolated constituting chromophores F and

C1) absorption spectra of dendrimer G01.

Fig. 7 Emission spectra calculated according to model presented in

ref. 73 using parameters relevant to the model chromophore (F) for

different solvent reorganization energies (see legend, eV units).


decrease of the anisotropy value with increasing emission

wavelength (from B0.1 at 400 nm to B0.02 at 600 nm) is

observed in the case of heterochromophoric dendrimers of the

DOND series (Fig. 9). This can be attributed to the contribu-

tion of two types of fluorophores (namely the core chromo-

phore and the peripheral fluorophores) with different

individual anisotropies, the combination of which results in a

wavelength-dependent global anisotropy. Whatever the wave-

length, the anisotropy values are lower than that of their

constituting chromophoric units (see r values for F and C0

or C1 in Table 1).83 These low values for such large nano-

objects indicate that fast energy transfer also takes place in the

DONDs. For wavelengths higher than 550 nm, where emission

stems only from the peripheral chromophores (see Fig. 4 and

5), the anisotropy values are similar to that of homochromo-

phoric SONDs indicative of fast energy transfer involving the

peripheral chromophores. The larger values obtained at lower

wavelength can be related to the increasing contribution of the

core chromophore, whose emission is blue-shifted with respect

to that of peripheral fluorophores (Fig. 6).84

Another important feature is the much lower fluorescence

quantum yield of the dendrimers of the DOND series as

compared to those of the SOND series (Table 1).85 This could

be related to interactions between chromophores giving rise to

competing processes such as fast energy transfer between

adjacent chromophores and formation of (red-shifted) exci-

mers or hetero-excimer with lower fluorescence quantum yield.

This is consistent with the rise of a red tail in the emission band

which is even broader than in the case of the SOND dendri-

mers (Fig. 8). Strikingly dendrimer G01 which is the less

fluorescent of the DOND series shows the more pronounced

red tail. Also, the larger disorder (as testified by the inhomo-

geneous broadening) within decorating fluorophores—due to

the different geometry of the dendrimers of the DOND series

as compared to the SOND series-can be responsible for

different overlap and interactions between the chromophores

on the periphery.

Two-photon absorption

The TPA of the dendrimers was studied by investigating the

TPEF of SOND and DOND dendrimers in toluene. TPEF

measurements allow for direct measurement of the TPEF

action cross-section s2F, the relevant figure of merit for

imaging applications.

The TPA spectra of dendrimers C0 and C1 were determined

as reference for the TPA response of core chromophores in the

DOND series. These model chromophores clearly show much

lower TPA cross-section than decorating fluorophore F over

the whole spectral range (Fig. 10) and their maximum TPA

cross-sections are more than five times smaller (Table 1). As a

Fig. 8 Comparison of the emission spectra of corresponding spherical-type and dumbbell-like nanodots in toluene.

Fig. 9 Emission anisotropy of monochromophoric dendrimers

C0–C1 and dumbbell-like nanodots G00–G02 in toluene (lexc =

382 nm).

Fig. 10 Two-photon absorption spectra of model fluorophore F and

monochromophoric dendrimers C0–C1 determined by femtosecond

TPEF measurements in toluene.


result, the TPA response of dendrimers of the DOND series is

expected to be dominated by the contribution of decorating

fluorophores given their larger molecular TPA response and

large excess (with a F/C ratio of, respectively 10/1, 20/1 and

40/1 for G00, G01 and G02). In addition the TPA response of

reference compounds C0 and C1 over 750 nm is clearly

negligible with respect to that of fluorophore F. Hence

although selective one-photon excitation of decorating fluoro-

phores of the DOND series was not permitted (similar absorp-

tion coefficient and almost complete overlap of the low energy-

absorption bands), selective two-photon excitation of the

decorating fluorophores of heterochromophoric DOND den-

drimers becomes possible. This is a major advantage of TP

excitation.

The TPA spectra of dendrimers of the SOND and DOND

series are shown in Fig. 11. The TPA cross-section in each

series of dendrimers increases almost linearly with the number

of decorating fluorophores leading to giant TPA cross-sec-

tions. This is apparent from the quasi overlap of the TPA

spectra normalized by the number of decorating chromo-

phores in the 700–740 nm region (Fig. 12).86 We observe

however a noticeable difference in the TPA spectra of den-

drimers of the SOND87 and DOND series at wavelengths

higher than 740 nm. In that spectral range, dendrimers of the

DOND series show definitely higher TPA cross-sections than

dendrimers of the SOND series. For example at 760 nm,

dendrimers of the DOND series show a TPA cross-section

normalized by the number of chromophores which is 60%

larger than dendrimers of the SOND series. This indicates that

the decorating fluorophores (or at least some of them) of the

DOND dendrimers show a definitely larger TPA cross-section

in the 750–850 nm region. This difference between DOND and

SOND nanostructures can be directly related to their different

topology and thus to the presence of the core chromophore.

Hence although the core chomophores (C0 or C1) show much

smaller TPA activity than decorating fluorophores (Fig. 10),

their presence affects the TPA response of peripheral fluoro-

phores. This can possibly be attributed to a change of

arrangement (relative orientation, packing) of decorating

fluorophores due to the different topology of the dendritic

architecture or to direct interactions between the core and

decorating fluorophores located close to it. Hence, the present

study demonstrates that the different topology of the dendri-

mers of the SOND and DOND series modifies both the PL

and TPA properties of the organic nanodots, providing evi-

dence that arrangement and/or environment of the two-

photon fluorophores in the organic nanodots can significantly

affect their response.

Conclusion

The present study demonstrates that the topology and nature

of the dendritic platform considerably influences the PL and

TPA properties of the organic nanodots. For comparable

numbers of decorating two-photon fluorophores, the PL

efficiency of the DOND dendrimers is much poorer due most

probably to interactions between the core chromophore and

proximal excited fluorophores (combined with fast energy

transfer which allows excitation energy migration towards

those traps). In contrast, the TPA efficiency is increased for

the DOND dendrimers as compared to the SOND dendrimers

in a large portion of the investigated spectral range (740–900

nm). Such phenomenon may have different causes. It might be

related to the change in topology caused by the different

geometry of the core, thus resulting in a different packing

and self-orientation of the decorating two-photon fluoro-

phores on the surface of the nanodots. Such effect can indeed

lead to changes of the TPA response of decorating fluoro-

phores as compared to isolated ones.66 Alternatively, the

change in TPA response could be due to a direct interaction

of the core with its proximal decorating fluorophores. Repla-

cing the chromophoric core by a non-conjugated core having

the same shape would allow to discriminate these causes. In

any case, this raises an important question since the global

TPA efficiency is found to be different for the two series. This

opens an interesting route towards improved TPA efficiency of

Fig. 11 Two-photon absorption spectra of SOND (G1–G4) and

DOND (G00–G02) dendrimers determined by femtosecond TPEF

measurements in toluene.

Fig. 12 Comparison of the TPA efficiency (normalized by the

number of TP fluorophores per nanodot) of SOND and DOND.

Inset: zoom of the 720–860 nm region (in the same units).


the nanodots by controlling the relative orientation of the

decorating fluorophores and or their environment.

We have also demonstrated in this study that all-organic

nanodots can exhibit two-photon absorption and two-photon

brightness comparable to the highest values reported for

quantum dots58 and represent thus a promising complemen-

tary approach towards fluorescent labels for TPEF imaging

applications. The highly modular design of nanodots allows

adjunction of supplementary layers, decoration of the peri-

phery with hydrosolubilizing groups, and insertion of func-

tional groups that could be utilized for bioconjugation

purposes. In that way, we have very recently reported71

biocompatible water-soluble monochromophoric nanodots

with shielding dendrimeric layers and peripheral ammonium

groups, that maintain their quantum efficiency in water and in

physiological environment, and the efficiency of which as

contrast agents has been demonstrated for in vivo TPEF

imaging on living animal.

Experimental

General

All manipulations were carried out with standard high vacuum

and dry-argon techniques. The solvents were freshly dried and

distilled (THF and ether over sodium/benzophenone, pentane

and CH2Cl2 over phosphorus pentoxide). Reactions were

monitored by thin layer chromatography on Merck silica gel

60 F254 precoated aluminum sheets. Column chromatogra-

phy: SDS silica gel Si 60 A CC (70–200 mm, 230–400 mesh) or

Florisils (60–100 mesh). Classical 1H, 13C, 31P NMR spectra

were recorded with Bruker AC 200, AC 250, or Avance 300,

500 spectrometers. References for NMR chemical shifts are

85% H3PO4 for31P NMR, SiMe4 for

1H and 13C NMR. The

attribution of 13C NMR signals has been done using Jmod, two

dimensional HMBC, and HMQC, Broad Band or CW 31P

decoupling experiments when necessary. The multiplicity of13C NMR signals is indicated only when the signal is not a

singlet. The numbering used for the assignment of NMR

signals is shown in Fig. 13.

Syntheses

Compound 1. Sodium hydride (0.18 g, 7.04 mmol) and dry

THF (20 mL) were introduced into a 50 mL Schlenk. 4-

Hydroxyanisole (0.74 g, 5.91 mmol) was added to this suspen-

sion. Once the solution was clear, it was cooled to 0 1C in a

water–ice bath and hexachlorocyclotriphosphazene (0.40 g,

1.14 mmol) was added at once. The reaction was stirred while

allowing the temperature to rise slowly to room temperature.

The reaction was considered to be finished after 40 h

(31P NMR monitoring), then the salts were centrifuged, and

the solvent was removed. The pentasubstituted core was

separated from the tetra- and hexasubstituted analogues by

column chromatography on silica gel using mixtures of hex-

anes and ethyl acetate of increasing polarity (9.1 to 8.2). The

pure product (0.50 g, 57% yield) was obtained as a colorless

oil.1H NMR (CDCl3, 300.1 MHz), d (ppm) = 3.78 (s, 15H,

OCH3), 6.72 (d, J = 9.0 Hz, 2H, C3A0 –H), 6.73 (d, J = 9.0 Hz,

4H, C3B0 –H), 6.79 (d, J = 9.0 Hz, 4H, C3B

0 –H), 6.87 (dd, J =

9.0 Hz/J = 1.8 Hz, 2H, C2A0 –H), 6.92 (d, J = 9.0 Hz, 4H,

C2B0 –H), 7.06 (d, J = 9.0 Hz, 4H, C2B

0 –H). 31P{1H} NMR

(CDCl3, 121.5 MHz), d (ppm) = 8.07 (d, J = 81.5 Hz, PB0 ),

8.08 (d, J= 78.0 Hz, PB0 ), 23.10 (dd, J= 78.0 Hz/J=81.5 Hz,

PA0 ).

13C NMR (CDCl3, 75.5 MHz), d (ppm) = 157.04 (d, J=

2.26 Hz, C4A0 ), 156.87 (C4B

0 ), 156.74 (C4B0 ), 144.01 (pseudo t,

J= 3.81 Hz, C1B0 ), 143.85 (pseudo t, J= 3.81 Hz, C1B

0 ), 143.75

(pseudo d, J = 9.81 Hz, C1A0 ), 122.15 (pseudo t, J = 2.26 Hz,

C2B0 ), 122.09 (m, C2A

0 ), 121.84 (pseudo t, J = 2.26 Hz, C2B0 ),

114.39 (C30), 55.53 (OCH3), 55.51 (OCH3).

Model compound C0. The pentasubstituted cyclotriphospha-

zene 1 (0.11 g, 0.14 mmol) and dry THF (2.5 mL) were

introduced into a 50-mL Schlenk. The biphenolic chromo-

phore 271 (0.036 g, 0.07 mmol) and caesium carbonate (0.05 g,

0.14 mmol) were added to this solution, and the resulting

mixture was stirred at room temperature for two days, then

five more days at 50 1C (31P NMR monitoring). The salts were

centrifuged off and the solvent was removed by vacuum

evaporation. The pure product (0.10 g, 55% yield) was

obtained after a column chromatography on silica gel using

mixtures of hexane–ethyl acetate of increasing polarity as

eluent (9 : 1 - 8 : 2 - 7 : 3 - 5 : 5).1H NMR (CDCl3, 300.1 MHz), d (ppm) = 0.63–0.75 (m,

10H, Hq, Ho), 1.14 (m, 4H, Hp), 2.07 (br m, 4H, Hn), 3.76 (s,

12H, CH3O), 3.80 (s, 18H, CH3O), 6.69–6.75 (m, 20H, C30–H),

6.84–6.93 (m, 20H, C20–H), 6.98 (d, J = 8.6 Hz, 4H, Hb), 7.16

(m, 4H, He, Hf), 7.37 (d, J= 8.6 Hz, 4H, Hc), 7.50 (s, 2H, Hk),

7.53 (d, J = 8.6 Hz, 2H, Hh), 7.71 (d, J = 7.8 Hz, 2H, Hi).31P{1H} NMR (CDCl3, 121.5 MHz), d (ppm) = 9.86 (m, P0).13C{1H} NMR (CDCl3, 62.9 MHz), d (ppm) = 13.89 (Cq),

23.14 (Cp), 26.03 (Co), 40.44 (Cn), 54.97 (Cm), 55.53 (OCH3),

114.34 (C30), 120.01 (Ci), 120.67 (Ck), 121.34 (m, Cb), 121.92

(m, C20), 125.66 (Ch), 127.02 (Ce), 127.33 (Cc), 129.16 (Cf),

134.22 (Cg), 136.24 (Cd), 140.71 (Cj), 144.30 (m, C10), 150.07

(m, Ca), 151.60 (Cl), 156.56 (C40).

Model compound C1. The first generation dendrimer 471

with the chromophore immobilized in the core (0.038 g, 10.68

mmol), 4-hydroxyanisole (0.030 g, 240.04 mmol), caesium

Fig. 13 Numbering used for NMR assignments (only half of the molecule is represented).


carbonate (0.150 g, 457.00 mmol), and an acetone–THF (1 : 1)

mixture (8 mL) were introduced in a 50-mL Schlenk. The

resulting heterogeneous solution was stirred at 50 1C for 20 h.

The salts were filtered and the solvent removed when the

reaction was finished (31P NMR monitoring). The crude

product was dissolved in minimum amount of chloroform

and added dropwise to 300 mL of pentane under vigorous

stirring, and the precipitated product recovered by filtration

(for three times) to afford the pure chromophoric core model

C1 (0.051 g, 89% yield).1H NMR (CDCl3, 300.1 MHz), d (ppm) = 0.53–0.69 (m,

10H, Hq, Ho), 1.09 (m, 4H, Hp), 2.02 (br m, 4H, Hn), 3.25 (d,3JH�P = 10.2 Hz, 30H, CH3N), 3.70 (s, 20H, CH3O), 3.72 (s,

10H, CH3O), 3.73 (s, 30H, CH3O), 6.78 (d, J = 9.0 Hz, 40H,

C31–H), 6.99–7.16 (m, 68H Hc, He, Hf, C

20–H, C2

1–H), 7.36–7.44

(m, 8H, Hh, Hk, Hb), 7.56–7-66 (m, 32H, Hi, C30–H, CHQN).

31P{1H} NMR (CDCl3, 121.5 MHz), d (ppm) = 8.47 (m, P0),

64.30 (s, P1), 64.40 (s, P1).13C{1H} NMR (CDCl3, 62.9 MHz),

d (ppm) = 13.91 (Cq), 23.08 (Cp), 26.00 (Co), 33.11 (d, JC�P =

12.1 Hz, CH3N), 40.69 (Cn), 55.00 (Cm), 55.52 (OCH3), 114.49

(C31), 120.06 (Ci), 120.67 (Ck), 121.12 (m, Cb), 121.41 (m, C2

0),

122.28 (d, JC�P1 = 4.4 Hz, C21), 125.54 (Ch), 126.73 (Ce),

127.50 (Cc), 129.41 (C30), 129.41 (Cf), 132.24 (m, C4

0), 134.72

(Cg), 136.02 (Cd), 138.28 (d, JC�P1 = 13.8 Hz, C0HQN),

140.72 (Cj), 144.14 (d, JC�P1 = 6.92 Hz, C11), 149.57 (m, Ca),

151.24 (m, C10), 151.56 (Cl), 156.98 (C4

1).

Dendrimer G00. The zeroth generation dendrimer 371 (0.005

g, 4.40 mmol), the terminal chromophore F61 (0.039 g, 46.34

mmol), caesium carbonate (0.029, 89.03 mmol), and a 1 : 1

tetrahydrofuran–acetone mixture (6 mL) were introduced in a

50 mL Schlenk, and stirred at 45 1C for 20 days, then for 17

days at 60 1C. The salts were filtered off and the solvent was

removed when the reaction was finished (31P NMR monitor-

ing). The product was purified by washing the orange solid

with a 1 : 1 acetone–methanol mixture to afford the orange

bichromophoric product (0.0182 g, 45% yield).1H NMR (CDCl3, 500.3 MHz), d (ppm) = 0.60–0.64 (m,

44H, HS, Ho), 0.65–0.70 (m, 66H, HU, Hq), 0.94 (br s, 60H,

HZ0), 1.10 (m, 44H, HT, Hp), 1.22 (m, 30H, HG), 1.36 (m,

120H, HX, HY, HZ), 1.62 (m, 40H, HW), 1.99 (m, 44H, HR,

Hn), 3.31 (br s, 40H, HV), 3.42–3.57 (m, 20H, HF), 3.63–3.79

(m, 20H, HE), 3.97–4.14 (m, 20H, HD), 6.61 (d, J = 7.6 Hz,

20H, HQ), 6.65 (m, 20H, HH), 6.70 (d, J = 7.4 Hz, 20H, HC),

6.91 (m, 20H, HB), 7.00 (m, 4H, Hb), 7.17 (m, 4H, He, Hf), 7.44

(m, 44H, HI, HP, Hc), 7.46–7.52 (m, 44H, HJ, HL, HN, HO, Hh,

Hk), 7.63 (m, 22H, HK, HM, Hi).31P{1H} NMR (CDCl3, 121.5

MHz), d (ppm) = 9.68 (m, P0).13C{1H} NMR (CDCl3, 125.8

MHz), d (ppm) = 12.28 (CG), 13.88 (CU, Cq), 14.08 (CZ0),

22.71 (CZ), 23.12 (CT, Cp), 25.89 (CS, Co), 26.84 (CX), 27.23

(CW), 31.74 (CY), 40.25 (Cn), 40.34 (CR), 45.63 (CF), 49.59

(CE), 50.98 (CV), 55.06 (CR0, Cm), 65.82 (CD), 88.22 (CN00),

88.61 (CI00 0), 90.78 (CI00), 91.22 (CN00 0), 108.76 (CP0), 110.07

(CI0), 111.23 (CQ), 111.46 (CH), 114.97 (CC), 119.73 (CK, CM),

120.09 (Ci), 120.71 (Ck), 121.24 (m, Cb), 121.97 (m, CB), 122.51

(CO00), 122.80 (CK0), 125.54 (CL, CO, Ch), 126.89 (Ce), 127.38

(Cc), 129.17 (Cf), 130.37 (CJ, CN), 132.89 (CP), 133.03 (CI),

134.31 (Cg), 136.12 (Cd), 136.12 (Cd), 140.00 (CN0), 140.22

(CL0), 140.72 (Cj), 144.56 (m, CB0), 147.37 (CH0), 147.94 (CQ0),

150.12 (m, Ca), 150.93 (CK00, CO0), 151.71 (Cl), 155.65 (CC0).

Dendrimer G01. The first generation dendrimer 471 (0.010 g,

2.83 mmol), the terminal chromophore F61 (0.049 g, 58.2

mmol), caesium carbonate (0.038, 113.94 mmol) and tetrahy-

drofuran (7 mL) were introduced in a 50 mL Schlenk, and

stirred at room temperature overnight then for 6 days at 45 1C.

The salts were filtered off and the solvent removed when the

reaction was finished (31P NMRmonitoring). The product was

purified by column chromatography (Florisils, CHCl3 to

THF–CH3OH as eluent mixtures) and further diethyl ether

washings to afford the orange bichromophoric product

(0.0191 g, 34% yield).1H NMR (CDCl3, 500.3 MHz), d (ppm) = 0.57–0.63 (m,

84H, HS, Ho), 0.63–0.70 (m, 126H, HU, Hq), 0.86–0.96 (m,

120H, HZ0), 1.04–1.12 (m, 84H, HT, Hp), 1.12–1.23 (m, 60H,

HG), 1.30–1.39 (m, 240H, HX, HY, HZ), 1.52–1.67 (m, 80H,

HW), 1.91–2.06 (m, 84H, HR, Hn), 3.18–3.30 (m, 30H,

P1–N–CH3), 3.27–3.36 (m, 80H, HV), 3.37–3.49 (m, 40H,

HF), 3.60–3.74 (m, 40H, HE), 3.92–4.06 (m, 40H, HD),

6.53–6.69 (m, 80H, HH, HQ), 6.69–6.83 (m, 40H, HC),

7.02–7.16 (m, 68H, HB, C20–H, Hb, He, Hf), 7.37–7.44 (m,

84H, HI, HP, Hc), 7.44–7.51 (m, 84H, HJ, HL, HN, HO, Hh,

Hk), 7.48–7.53 (m, 30H, C30–H, CHQN), 7.58–7.69 (m, 42H,

HK, HM, Hi).31P{1H} NMR (CDCl3, 202.5 MHz), d (ppm) =

8.58 (m, P0), 64.58 (m, P1).13C{1H} NMR (CDCl3, 125.8

MHz), d (ppm) = 12.22 (CG), 13.86 (CU, Cq), 14.05 (CZ0),

22.68 (CZ), 23.09 (CT, Cp), 25.88 (CS, Co), 26.80 (CX), 27.17

(CW), 31.70 (CY), 33.05 (m, P1–N–CH3), 40.30 (CR, Cn), 45.60

(CF), 49.50 (CE), 50.99 (CV), 55.05 (CR0, Cm), 65.69 (CD), 88.54

(CI00 0, CN00), 91.03 (CI00, CN00 0), 108.81 (CP0), 109.91 (CI0), 111.41

(CH, CQ), 115.04 (CC), 119.72 (CK, CM), 121.36 (m, C20, Cb, Ci,

Ck), 122.37 (m, CB), 122.75 (CK0, CO00), 125.53 (CL, CO, Ch),

127.17 (m, Cc, Ce), 128.38 (C30), 129.01 (Cf), 130.37 (CJ, CN),

132.17 (m, C40), 132.88 (CP, CI), 133.81 (Cg), 136.14 (Cd),

140.17 (m, CL0, CN0, C0HQN, Cj), 144.27 (m, CB0), 147.33

(CH0), 147.85 (CQ0), 150.93 (m, CK00, CO0, C10, Ca, Cl), 156.03

(CC0).

Dendrimer G02. The second generation dendrimer 571 (0.078

g, 0.91 mmol), the terminal chromophore F61 (0.031 g, 36.48

mmol), caesium carbonate (0.026 g, 78.09 mmol) and a 1 : 1

acetone–THF mixture (6 mL) were introduced in a 50-mL

Schlenk, and stirred at 50 1C for 21 h. The salts were filtered

and the solvent removed when the reaction was finished (31P

NMR monitoring). The orange solid obtained was repeatedly

washed with diethyl ether until no F chromophore was ob-

served by TLC. Then the solid was taken in dichloromethane,

filtered, and the solvent was removed to afford the orange

bichromophoric product (0.034 g, 91% yield).1H NMR (CDCl3, 500.3 MHz), d (ppm) = 0.50–0.74 (m,

410H, HS, HU, Ho, Hq), 0.85–0.96 (m, 240H, HZ0), 0.99–1.19

(m, 284H, HG, HT, Hp), 1.24–1.29 (m, 480H, HX, HY, HZ),

1.52–1.67 (m, 160H, HW), 1.85–2.09 (m, 164H, HR, Hn),

3.12–3.49 (m, 330H, HF, HV, P1-N-CH3, P2-N-CH3),

3.53–3.72 (m, 80H, HE), 3.87–4.07 (m, 80H, HD), 6.52–6.67

(m, 160H, HH,, HQ), 6.68–6.81 (m, 80H, HC), 7.00–7.25 (m,

148H, HB, C20–H, C2

1–H, Hb, He, Hf), 7.34–7.44 (m, 164H, HI,


HP, Hc), 7.44–7.54 (m, 194H, HJ, HL, HN, HO, C0HQN,

C1HQN, Hh, Hk), 7.54–7.72 (m, 142H, HK, HM, C30–H, C3

1–H,

Hi).31P{1H} NMR (CDCl3, 121.5 MHz), d (ppm) = 8.43 (m,

P0), 62.34 (br s, P1), 64.62 (br s, P2).13C{1H} NMR (CDCl3,

125.8 MHz), d (ppm) = 12.24 (CG), 13.87 (CU, Cq), 14.07

(CZ0), 22.70 (CZ), 23.09 (CT, Cp), 25.88 (CS, Co), 26.82 (CX),

27.22 (CW), 31.72 (CY), 33.02 (m, P1–N–CH3, P2–N–CH3),

40.30 (CR, Cn), 45.59 (CF), 49.51 (CE), 50.96 (CV), 55.04 (CR0,

Cm), 65.72 (CD), 88.22 (CN00), 88.54 (CI00 0), 90.87 (CI00), 91.23

(CN00 0), 108.74 (CP0), 109.87 (CI0), 111.22 (CQ), 111.40 (CH),

115.03 (CC), 119.72 (CK, CM), 121.86 (m, C20, C

21, Cb, Ci, Ck),

122.42 (m, CB), 122.78 (CK0, CO00), 125.51 (CL, CO, Ch), 127.11

(Cc, Ce), 128.38 (C30, C

31), 129.05 (Cf), 130.36 (CJ, CN), 131.59

(C41), 132.34 (C4

0), 132.88 (CP), 132.98 (CI), 133.67 (Cg), 136.18

(Cd), 139.98 (CN0), 140.17 (m, CL0, CHQN, Cj), 144.25 (m,

CB0), 147.33 (CH0), 147.93 (CQ0), 150.92 (m, CK00, CO0, C10, Ca,

Cl), 151.25 (m, C11), 156.01 (CC0).

Photophysical studies

Optical absorption and emission spectroscopy. All photophy-

sical measurements have been performed with freshly-pre-

pared solutions in air-equilibrated toluene at room

temperature (298 K). UV/Vis absorption spectra were re-

corded on a Jasco V-570 spectrophotometer. Steady-state

and time-resolved fluorescence measurements were performed

on dilute solutions (ca. 10�6 M chromophore concentration,

optical density o0.1) contained in standard 1 cm quartz

cuvettes using an Edinburgh Instruments (FLS920) spectro-

meter in photon-counting mode. Emission spectra were ob-

tained, for each compound, under excitation at the wavelength

of the absorption maximum. Fluorescence quantum yields

were measured according to literature procedures using fluor-

escein in 0.1 M NaOH as a standard (quantum yield F =

0.90).88,89 The lifetime values were obtained from the recon-

volution fit analysis (Edinburgh F900 analysis software) of

decay profiles obtained using the FLS920 instrument under

excitation with a nitrogen-filled nanosecond flash-lamp. The

quality of the fits was evidenced by the reduced w2 value

(w2 o1.1).

Fluorescence anisotropy. Steady-state fluorescence anisotro-

pies were measured using the Edinburgh Instruments FLS920

instrument, by inserting Glan–Thompson polarizers in the

excitation and emission paths. Emission anisotropies were

obtained at right angles using vertically polarized excitation

light. Wavelength-dependent polarization correction factors

(‘‘G-factors’’) were measured on the same sample under

horizontally polarized excitation following standard proce-

dures.

Two-photon absorption. Two-photon absorption cross sec-

tions (s2) were obtained from the two-photon excited fluores-

cence (TPEF) cross sections (s2F) and the fluorescence

emission quantum yield (F). TPEF cross sections in toluene

(10�4 M chromophore concentration) were determined using a

Ti-sapphire laser delivering 150 fs excitation pulses, according

to the experimental protocol established by Xu and Webb.90

This experimental protocol allows avoiding contributions

from excited-state absorption that are known to result in

largely overestimated TPA cross-sections. Fluorescein in 0.01

M NaOH, whose TPEF cross-sections are well-known,90

served as the reference, taking into account the necessary

corrections for the refractive index of the solvents.91 The

quadratic dependence of the fluorescence intensity on the

excitation intensity was verified for each data point, indicating

that the measurements were carried out in intensity regimes in

which saturation or photodegradation do not occur. More

details about the experimental setup have been previously

published.91

Acknowledgements

F. T. acknowledges Universite de Rennes 1 for an invited

assistant professor position. We acknowledge financial sup-

port from CNRS, Region Bretagne (‘‘Renouvellement des

Competences’’ Program) and Rennes Metropole. Thanks are

due to the Fundacion Ramon Areces for a grant to A.

Pla-Quintana.

References

1 R. R. Birge, Acc. Chem. Res., 1986, 19, 138–146.2 S. Shima, R. P. Ilagan, N. Gillespie, B. J. Sommer, R. G. Hiller, F.P. Sharples, H. A. Frank and R. R. Birge, J. Phys. Chem. A, 2003,107, 8052–8066.

3 D. A. Parthenopoulos and P. M. Rentzepis, Science, 1989, 245,843–845.

4 J. H. Strickler and W. W. Webb, Opt. Lett., 1991, 16, 1780–1782.5 A. S. Dvornikov and P. M. Rentzepis, Opt. Commun., 1995, 119,341–346.

6 S. Maruo, O. Nakamura and S. Kawata, Opt. Lett., 1997, 22,132–134.

7 B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E.Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler, I.-Y. S. Lee,D. McCord-Maughon, J. Qin, H. Rockel, M. Rumi, X. L. Wu, S.R. Marder and J. W. Perry, Nature, 1999, 398, 51–54.

8 S. Kawata, H.-B. Sun, T. Tanaka and K. Takada, Nature, 2001,412, 697–698.

9 J. D. Bhawalkar, G. S. He, C.-K. Park, C. F. Zhao, G. Ruland andP. N. Prasad, Opt. Commun., 1996, 124, 33–37.

10 A. Abbotto, L. Beverina, R. Bozio, S. Bradamante, C. Ferrante, G.A. Pagani and R. Signorini, Adv. Mater., 2000, 12, 1963–1967.

11 W. Denk, J. H. Strickler and W. W. Webb, Science, 1990, 248,73–76.

12 C. Xu, W. Zipfel, J. B. Shear, R. M. Williams and W. W. Webb,Proc. Natl. Acad. Sci. USA, 1996, 93, 10763–10768.

13 W. R. Zipfel, R. M. Williams, R. Christie, A. Y. Nikitin, B. T.Hyman and W. W. Webb, Proc. Natl. Acad. Sci. USA, 2003, 100,7075–7080.

14 J. D. Bhawalkar, N. D. Kumar, C. F. Zhao and P. N. Prasad, J.Clin. Laser Med. Surg., 1997, 15, 201–204.

15 G. S. He, J. D. Bhawalkar, C. F. Zhao and P. N. Prasad, Appl.Phys. Lett., 1995, 67, 2433–2435.

16 G. S. He, L. Yuan, N. Cheng, J. D. Bhawalkar, P. N. Prasad, L. L.Brott, S. J. Clarson and B. A. Reinhardt, J. Opt. Soc. Am. B, 1997,14, 1079–1087.

17 B. A. Reinhardt, L. L. Brott, S. J. Clarson, A. G. Dillard, J. C.Bhatt, R. Kannan, L. Yuan, G. S. He and P. N. Prasad, Chem.Mater., 1998, 10, 1863–1874.

18 K. D. Belfield, D. J. Hagan, E. W. Van Stryland, K. J. Schafer andR. A. Negres, Org. Lett., 1999, 1, 1575–1578.

19 Z.-q. Liu, Q. Fang, D. Wang, D.-x. Cao, G. Xue, W.-t. Yu and H.Lei, Chem. Eur. J., 2003, 9, 5074–5084.

20 S. Charier, O. Ruel, J.-B. Baudin, D. Alcor, J.-F. Allemand, A.Meglio and L. Jullien, Angew. Chem., Int. Ed., 2004, 43,4785–4788.

21 G. S. He, G. C. Xu, P. N. Prasad, B. A. Reinhardt, J. C. Bhatt, R.McKellar and A. G. Dillard, Opt. Lett., 1995, 20, 435–437.


22 J. E. Ehrlich, X. L. Wu, I. Y. S. Lee, Z. Y. Hu, H. Rockel, S. R.Marder and J. W. Perry, Opt. Lett., 1997, 22, 1843–1845.

23 M. Albota, D. Beljonne, J.-L. Bredas, J. E. Ehrlich, J.-Y. Fu, A. A.Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder, D.McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi, G. Sub-ramaniam, W. W. Webb, X.-L. Wu and C. Xu, Science, 1998, 281,1653–1656.

24 L. Ventelon, L. Moreaux, J. Mertz and M. Blanchard-Desce,Chem. Commun., 1999, 2055–2056.

25 O.-K. Kim, K.-S. Lee, H. Y. Woo, K.-S. Kim, G. S. He, J.Swiatkiewicz and P. N. Prasad, Chem. Mater., 2000, 12,284–286.

26 L. Ventelon, S. Charier, L. Moreaux, J. Mertz and M. Blanchard-Desce, Angew. Chem., Int. Ed., 2001, 40, 2098–2101.

27 P. K. Frederiksen, M. Jørgensen and P. R. Ogilby, J. Am. Chem.Soc., 2001, 123, 1215–1221.

28 O. Mongin, L. Porres, L. Moreaux, J. Mertz and M. Blanchard-Desce, Org. Lett., 2002, 4, 719–722.

29 A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante, G.A. Pagani, D. Pedron and R. Signorini, Org. Lett., 2002, 4,1495–1498.

30 W. J. Yang, D. Y. Kim, M.-Y. Jeong, H. M. Kim, S.-J. Jeon and B.R. Cho, Chem. Commun., 2003, 2618–2619.

31 M. H. V. Werts, S. Gmouh, O. Mongin, T. Pons and M. Blan-chard-Desce, J. Am. Chem. Soc., 2004, 126, 16294–16295.

32 K. D. Belfield, A. R. Morales, B.-S. Kang, J. M. Hales, D. J.Hagan, E. W. Van Stryland, V. M. Chapela and J. Percino, Chem.Mater., 2004, 16, 4634–4641.

33 Z.-Q. Liu, Q. Fang, D.-X. Cao, D. Wang and G.-B. Xu,Org. Lett.,2004, 6, 2933–2936.

34 S. K. Lee, W. J. Yang, J. J. Choi, C. H. Kim, S.-J. Jeon and B. R.Cho, Org. Lett., 2005, 7, 323–326.

35 S.-J. Chung, M. Rumi, V. Alain, S. Barlow, J. W. Perry and S. R.Marder, J. Am. Chem. Soc., 2005, 127, 10844–10845.

36 H. Y. Woo, B. Liu, B. Kohler, D. Korystov, A. Mikhailovsky andG. C. Bazan, J. Am. Chem. Soc., 2005, 127, 14721–14729.

37 V. Strehmel, A. M. Sarker, P. M. Lahti, F. E. Karasz, M.Heydenreich, H. Wetzel, S. Haebel and B. Strehmel, Chem-PhysChem, 2005, 6, 267–276.

38 O. Mongin, L. Porres, M. Charlot, C. Katan and M. Blanchard-Desce, Chem. Eur. J., 2007, 13, 1481–1498.

39 M. P. Joshi, J. Swiatkiewicz, F. Xu, P. N. Prasad, B. A. Reinhardtand R. Kannan, Opt. Lett., 1998, 23, 1742–1744.

40 S.-J. Chung, K.-S. Kim, T.-C. Lin, G. S. He, J. Swiatkiewicz and P.N. Prasad, J. Phys. Chem. B, 1999, 103, 10741–10745.

41 G. S. He, J. Swiatkiewicz, Y. Jiang, P. N. Prasad, B. A. Reinhardt,L.-S. Tan and R. Kannan, J. Phys. Chem. A, 2000, 104, 4805–4810.

42 B. R. Cho, K. H. Son, S. H. Lee, Y.-S. Song, Y.-K. Lee, S.-J. Jeon,J. H. Choi, H. Lee and M. Cho, J. Am. Chem. Soc., 2001, 123,10039–10045.

43 D. Beljonne, W. Wenseleers, E. Zojer, Z. Shuai, H. Vogel, S. J. K.Pond, J. W. Perry, S. R. Marder and J.-L. Bredas, Adv. Funct.Mater., 2002, 12, 631–641.

44 A. Adronov, J. M. J. Frechet, G. S. He, K.-S. Kim, S.-J. Chung, J.Swiatkiewicz and P. N. Prasad, Chem. Mater., 2000, 12,2838–2841.

45 S.-J. Chung, T.-C. Lin, K.-S. Kim, G. S. He, J. Swiatkiewicz, P. N.Prasad, G. A. Baker and F. V. Bright, Chem. Mater., 2001, 13,4071–4076.

46 A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante, G.A. Pagani, D. Pedron and R. Signorini, Chem. Commun., 2003,2144–2145.

47 O. Mongin, L. Porres, C. Katan, T. Pons, J. Mertz and M.Blanchard-Desce, Tetrahedron Lett., 2003, 44, 8121–8125.

48 J. Yoo, S. K. Yang, M.-Y. Jeong, H. C. Ahn, S.-J. Jeon and B. R.Cho, Org. Lett., 2003, 5, 645–648.

49 L. Porres, O. Mongin, C. Katan, M. Charlot, T. Pons, J. Mertzand M. Blanchard-Desce, Org. Lett., 2004, 6, 47–50.

50 C. Katan, F. Terenziani, O. Mongin, M. H. V. Werts, L. Porres, T.Pons, J. Mertz, S. Tretiak andM. Blanchard-Desce, J. Phys. Chem.A, 2005, 109, 3024–3037.

51 C. Le Droumaguet, O. Mongin, M. H. V. Werts and M.Blanchard-Desce, Chem. Commun., 2005, 2802–2804.

52 M. Drobizhev, A. Karotki, A. Rebane and C. W. Spangler, Opt.Lett., 2001, 26, 1081–1083.

53 M. Drobizhev, A. Karotki, Y. Dzenis, A. Rebane, Z. Suo and C.W. Spangler, J. Phys. Chem. B, 2003, 107, 7540–7543.

54 O. Mongin, J. Brunel, L. Porres and M. Blanchard-Desce, Tetra-hedron Lett., 2003, 44, 2813–2816.

55 O. Varnavski, X. Yan, O. Mongin, M. Blanchard-Desce and T.Goodson, III, J. Phys. Chem. C, 2007, 111, 149–162.

56 Corresponding to the combination of reduced absorption andscattering.

57 In relation with reduced excitation of endogenous chromophores.58 D. R. Larson, W. R. Zipfel, R. M. Williams, S. W. Clark, M. P.

Bruchez, F. W. Wise and W. W. Webb, Science, 2003, 300,1434–1437.

59 X. Michalet, F. F. Pinaud, L. A. Bentolila, J. M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A. M. Wu, S. S. Gambhir and S. Weiss,Science, 2005, 307, 538–544.

60 R. Hardman, Environ. Health Perspect., 2006, 114, 165–172.61 O. Mongin, T. R. Krishna, M. H. V. Werts, A.-M. Caminade, J.-P.

Majoral and M. Blanchard-Desce, Chem. Commun., 2006,915–917.

62 Phosphorus dendrimers have been shown to have low toxicity, see:M. Maszewska, J. Leclaire, M. Cieslak, B. Nawrot, A. Okruszek,A.-M. Caminade and J.-P. Majoral, Oligonucleotides, 2003, 13,193–205.

63 W. West and S. Pearce, J. Phys. Chem., 1965, 69, 1894–1903.64 G. De Belder, G. Schweitzer, S. Jordens, M. Lor, S. Mitra, J.

Hofkens, S. De Feyter, M. Van der Auweraer, A. Herrmann, T.Weil, K. Mullen and F. C. De Schryver, ChemPhysChem, 2001, 2,49–55.

65 F. Terenziani, M. Morone, S. Gmouh and M. Blanchard-Desce,ChemPhysChem, 2006, 7, 685–696.

66 G. D’Avino, F. Terenziani and A. Painelli, J. Phys. Chem. B, 2006,110, 25590–25592.

67 J.-P. Majoral and A.-M. Caminade, Chem. Rev., 1999, 99,845–880.

68 A.-M. Caminade and J.-P. Majoral, Acc. Chem. Res., 2004, 37,341–348.

69 A.-M. Caminade and J.-P. Majoral, Prog. Polym. Sci., 2005, 30,491–505.

70 N. Launay, A.-M. Caminade and J. P. Majoral, J. Organomet.Chem., 1997, 529, 51–58.

71 T. R. Krishna, M. Parent, M. H. V. Werts, L. Moreaux, S.Gmouh, S. Charpak, A.-M. Caminade, J.-P. Majoral and M.Blanchard-Desce, Angew. Chem., Int. Ed., 2006, 45, 4645–4648.

72 A slight apparent decrease of the effective absorption coefficientper chromophore is observed for dendrimers G3 and G4 (inferiorto that of reference fluorophore F) This is most probably anartifact related to the presence of an increasing amount of solventmolecules trapped in the dendrimer interior. A similar effect isconsequently observed for the maximum TPA cross-section of G3

and G4.73 F. Terenziani, A. Painelli, C. Katan, M. Charlot and M. Blan-

chard-Desce, J. Am. Chem. Soc., 2006, 128, 15742–15755.74 M. N. Berberan-Santos, J. Pouget, B. Valeur, J. Canceill, L. Jullien

and J. M. Lehn, J. Phys. Chem., 1993, 97, 11376–11379.75 T. Pullerits and V. Sundstrom, Acc. Chem. Res., 1996, 29, 381–389.76 R. van Grondelle, R. Monshouwer and L. Valkunas, Pure Appl.

Chem., 1997, 69, 1211–1218.77 E. K. L. Yeow, K. P. Ghiggino, J. N. H. Reek, M. J. Crossley, A.

W. Bosman, A. P. H. J. Schenning and E. W. Meijer, J. Phys.Chem. B, 2000, 104, 2596–2606.

78 M. Maus, S. Mitra, M. Lor, J. Hofkens, T. Weil, A. Herrmann, K.Mullen and F. C. De Schryver, J. Phys. Chem. A, 2001, 105,3961–3966.

79 J. Larsen, J. Andersson, T. Polivka, J. Sly, M. J. Crossley, V.Sundstrom and E. Akesson, Chem. Phys. Lett., 2005, 403, 205–210.

80 Time-resolved fluorescence anisotropy decay measurements studyon dendrimers G1–G4 indicates that this energy transfer is indeedvery fast: M. Guo and T. Goodson, III, personal communication.

81 T. Forster, Ann. Phys., 1948, 2, 55–75.82 A decrease of the fluorescence quantum yield accompanied by the

appearance of an emission tail at lower energy has also beenobserved in dendrimers decorated with naphthyl units: F. Pina,P. Passaniti, M. Maestri, V. Balzani, F. Vogtle, M. Gorka, S.-K.Lee, J. Van Heyst and H. Fakhrnabavi, ChemPhysChem, 2004, 5,473–480.


83 Model chromophore F exhibits an anisotropy of 0.18, and refer-ence monochromophoric compounds C0 and C1 display values of0.22 and 0.28, respectively. Since these two dendrimer-embeddedchromophores differ only in generation number, the increase inanisotropy value can be related to the slowing-down of therotational diffusion for increasing molecule size.

84 DOND G00 shows the smaller anisotropy value at low wavelength(0.07), which might indicate that energy transfer from core chro-mophore to peripheral ones also takes place in that case (in relationwith the smaller distance between core and peripheral chromo-phores). Wavelength-dependent time-resolved anisotropy measure-ments are required to derive final conclusion.

85 The fluorescence quantum yield is an effective value since both thecore and peripheral chromophores contribute to emission. Its valueis thus expected to depend on the excitation wavelength, whichcontrols the relative contribution (in addition to the relative

number of chromophores) of excitation of each type of chromo-phores. The values given in Table 1 correspond to excitation at theabsorption maximum.

86 Except for dendrimer G02 that shows slightly higher TPA cross-sections.

87 Even if their PL characteristics show that emitting fluorophores ofSOND dendrimers experience a more polar environment, theaverage TPA response of the decorating fluorophores is similarto that of an isolated molecule (Fig. 12), which was also the casefor one-photon absorption (Fig. 4).

88 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75,991–1024.

89 D. F. Eaton, Pure Appl. Chem., 1988, 60, 1107–1114.90 C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481–491.91 M. H. V. Werts, N. Nerambourg, D. Pelegry, Y. Le Grand and M.

Blanchard-Desce, Photochem. Photobiol. Sci., 2005, 4, 531–538.


organic nanodots for multiphotonics: synthesis and...

Documents